1. Field of the Invention
The invention relates to a stick packaging casing (concertina) based on polymer blends, and in particular to a synthetic, polyamide-based sausage casing which has been shirred from biaxially stretch-oriented and heat-set tube material.
2. Description of Related Art
Numerous plastic films for packaging foodstuffs which can be filled as a paste or melt are already known. As a rule, such plastic films are produced from polyethylene terephthalate (PET), polyvinylidene chloride (PVDC) and polyamide (PA). Biaxially stretched PET casings are, however, unsuitable for cooked and scalded sausages, since they are not very stretchable and they shrink only to a small extent. Although PVDC casings are substantially more stretchable, they generally lose their shape after filling more readily. This is particularly the case when sausages filled with these sort of casings are hung up for a prolonged period. Polyamide casings are more stretchable, shrink more and are therefore generally more suitable for cooked and scalded sausages.
DE-A 2 850 182 and GB-A 2 035 198 describe a single-layer casing, which has been shrinkably stretched in the longitudinal and transverse directions and heat-set. The casing is formed of an aliphatic polyamide having a glass transition point in the dry state of at least 48.degree. C., which is reduced at least to 3.degree. C. on absorption of moisture. Polyamide-6 (polycaprolactam), polyamide-7 poly(7-aminoheptanoic acid lactam) or poly(x-enanthic acid lactam)!, polyamide-6,6 (polyamide of hexamethylenediamine and adipic acid) and polyamide-6,10 polyamide of hexamethylenediamine and decanedioc acid (sebacic acid)! are actually disclosed as possible polyamides.
A similar casing is also described in DE-A 2 850 181 and GB-A 2 035 198. It is composed of a polymer blend which, in addition to the aliphatic polyamide, also contains an ionomer resin, a modified ethylene/vinyl acetate copolymer and/or a quaternary polymer with units of ethylene, butylene, an aliphatic, ethylenically unsaturated (C.sub.3 -C.sub.5)-carboxylic acid and an ester of this carboxylic acid with (C.sub.1 -C.sub.8)-alcohols. After initial cutting, it is disclosed that the casing shows a reduced tendency for tear propagation. These casings are used for cooked and scalded sausages. However, the sausage meat must be filled in under a pressure of from about 0.3 to 0.6 bar, in order to stretch the casing.
Since the sausage meat loses volume after scalding, the casing must compensate this loss, so that it fits closely without creases, i.e., it must shrink to the same extent as its contents. The known casings, however, are generally shrinkable only to a limited extent and are therefore stretched during the filling step. Thus, the necessary filling pressure is generally at the limit of what is possible with the known sausage-filling machines. In the case of manual filling, the pressure is so low that such casings would detach themselves from the sausage meat, in other words, the casing separates, and will form creases, which would cause the sausage to appear unattractive and no longer fresh in the eyes of the consumer.
A single-layer sausage casing of a polyamide, which can absorb at least 5% of its weight of water, is disclosed in DE-A 3 227 945, U.S. Pat. No. 4,560,520 and U.S. Pat. No. 4,601,929. It shows a reduced tendency to tear propagation after initial cutting than the sausage casing known from DE-A 2 850 182. Starting materials for this casing are preferably polyamide-6 and polyamide-6,6. It is produced by extrusion, stretching of the extruded tube in the longitudinal and transverse directions and subsequent complete heat-setting under controlled shrinkage. The controlled shrinkage amounts to 15% to about 40% in each direction, as a rule about 20%. Even here, the casing must be expanded by elevated pressure during filling, if it is to remain free of creases later.
The mechanical properties of stretched polyamide casings depend greatly on their water content. A minimum water content is generally necessary to keep the casings sufficiently stretchable and supple during filling operations. However, water and water vapor increase the water content during scalding to such an extent that the mechanical properties of the casings can become adversely affected.
A polyamide-based single-layer casing for cooked and scalded sausages, which retains its shape even during scalding, is disclosed in EP-A 0 176 980 and U.S. Pat. No. 4,659,599. It is composed of a mixture of polyamide and polyester. The polyamide is a saturated, linear, aliphatic polyamide, in particular PA-6, and the polyester is a polyterephthalic acid ester and/or a copolyester with units of terephthalic acid and isophthalic acid. Due to the admixture of polyester, the casing is relatively cloudy and shows an unnatural metallic sheen. In addition, its stretchability is low and it must be filled under a comparatively high pressure if it is to be free of creases.
The single-layer, biaxially stretch-oriented and heat-set sausage casing according to DE-A 3 943 024 and U.S. Pat. No. 5,326,613 is clear as glass, has no metallic or mother-of-pearl like sheen and shows a high resilience. It is produced from a polymer blend composed of polycaprolactam (polyamide-6) and an aromatic copolyamide of units of hexamethylenediamine, terephthalic and isophthalic acids. This casing is again most specifically intended for filling by machine under pressure. It is less suitable for manual filling, because in most cases it is not sufficiently supple.
In EP-A 0 573 306, a tubular, multi-layered, biaxially stretched, shrinkable sausage casing is described. The casing contains a layer of a polymer blend composed of polyamide-6 and at least 5% by weight of a copolyamide (for example PA-6,12 or PA-6/69). This layer is surrounded by other layers, especially by polyolefin layers. The polyamide layer is relatively thin as compared with the polyolefin layers. The foodstuffs casings actually disclosed in EP-A 0 573 306 all have six layers.